Aluminum nitride exhibits a variety of properties that makes it unique among ceramic materials. It has a low electrical conductivity (10.sup.-11 to 10.sup.-13 W.sup.-1 cm.sup.-1) and a moderately low dielectric constant and dielectric loss. At the same time it has a very high theoretical thermal conductivity (319-320 W/m.K). These properties make the material especially useful in applications such as semiconductor substrates, where beryllium oxide (BeO) has been extensively used in the past. The use of a high thermal conductivity electrical insulator for substrates is essential in the development of miniature electronics because the increased component density creates large amounts of waste heat that must be quickly dissipated through the substrate. Also the thermal expansion coefficient of AlN (2.64.times.10.sup.-6 K.sup.-1) is much lower than that of BeO (5.7.times.10.sup.-6 K.sup.-1) and alumina (7.2-8.6.times.10.sup.-6 K.sup.-1), and it closely matches that of silicon. Therefore, electronic devices supported by AlN are less likely to fail from thermal cycling than those supported by BeO or alumina substrates. The thermal conductivity of AlN, though slightly less than that of BeO at room temperature, is also less temperature sensitive than that of BeO and exceeds that of BeO above about 473 K.
Another potential application for AlN is as a packaging material for electronic items. Packages made of AlN could reduce cooling problems and allow higher power densities. AlN also shows corrosion resistance to a wide variety of materials. It is wetted by molten aluminum, but does not react with it. It is not attacked by uranium, lithium, many ferrous alloys and some superalloys. It is also stable against molten salts such as carbonate eutectic mixtures and cryolite. AlN is finding increased applications in crucibles and hardware for containing or processing many of these corrosive materials.
AlN also has a variety of structural and refractory applications because it exhibits the high strength and high temperature stability associated with most non-oxide materials. It has also been suggested for use as a filler in metals or polymers, to alter the properties of the matrix material. In metals, for example in aluminum, AlN can be used much like silicon carbide to stiffen and strengthen the matrix. Aluminum nitride has an added advantage over silicon carbide in this application because it does not react with the metal. This allows longer processing times for the composite in the molten form, as well as more control over the interface between the matrix and the filler. In polymers, AlN can be used to increase the stiffness of the polymer, to reduce the thermal expansion of the polymer or to boost the thermal conductivity of the polymer. High thermal conductivity polymers have a wide variety of applications, from sealants for electronic applications to heat-dissipating structural, adhesive or insulating materials.
Mellor (1928) described the early production of aluminum nitride (AlN), crediting Briegleb and Geuther (1862) as being the first to document a method for its production. Their method involved heating aluminum in a nitrogen atmosphere to yield (impure) AlN according to the nitridation reaction: EQU Al(s)+1/2N.sub.2 (g)=AlN(s) (1)
Mellor also reported the commercial production of AlN by Serpek via the carbothermal reduction of alumina using coal and bauxite as starting materials: EQU Al.sub.2 O.sub.3 (s)+3C(s)+N.sub.2 (g)=2AlN(s)+3CO(g) (2)
As reactions observed in AlN powder production, (1) and (2) above are by far the most prevalent (Fister, 1985). Purity has been increased over the years by increasing the purity of reactants and by reducing the particle size of the solid reactants.
U.S. Pat. No. 4,160,857 to Ogawa and Abe (1986) described a general method for manufacturing ceramic powders that utilized a plasma jet and produced particles less than 0.1 .mu.m in diameter. The process of Ogawa et al. was deemed to be advantageous in that the reaction occurred between gaseous species, which could result in potentially higher purity powders than those produced frown heterogeneous reactions, although no information concerning purity was provided. Details on the polydispersity of the products were also absent.
Shintaku (1986) produced AlN powder by the direct nitridation of liquid aluminum using gaseous nitrogen. Here, molten aluminum was atomized into N.sub.2 (g) which was at a minimum temperature of 1073 K. The AlN produced in the cited examples had a maximum purity of 60 wt % AlN (the remainder being Al) and an average particle size of 0.1-0.2 .mu.m. Further processing (heating at elevated temperatures under a N.sub.2 atmosphere) was required to achieve "substantially 100% aluminum nitride."
Hotta et al. (1987, 1988) applied the direct nitridation of liquid aluminum particles using NH.sub.3 (g) according to the reaction: EQU Al(1)+NH.sub.3 (g)=AlN(s)+3/2H.sub.2 (g) (4)
Hotta et al. manufactured hollow, spherical AlN particles over the size range 4-12 .mu.m, but this range was reduced to 0.1-0.2 .mu.m through milling. The hollow particles are reasoned to be the result of the kinetics of the reaction where it is believed that a nitride layer is formed around the molten aluminum, which expands more than the surrounding AlN layer, causing fissures in the surface through which the molten aluminum escapes. In the Hotta et al. reactor, the product powder had to be scraped repeatedly from the reactor walls with a hot tungsten wire in order to establish a continuous process.
Baba et al. (1989) utilized radio frequency (rf) plasma techniques, directly nitrided Al(g) with NH.sub.3 (g), and produced particles about 60 nm in diameter, with a total metallic content of 100 parts per million (ppm) as measured by X-ray diffraction (XRD). The Al content was not measurable.
Yoshimura et al. (1990) also used homogeneous, gas-phase nitridation to produce AlN powder. An arc image lamp was focused onto a solid block of aluminum to vaporize it. Both NH.sub.3 and nitrogen were tried as nitriding agents, and it was found that, while nitrogen does nitride the aluminum, the time required to achieve results similar to those using ammonia was nearly two orders of magnitude higher. The percent conversion varied from (approximately) 10% at a residence time of 1.5 seconds, to 95% at 20 seconds, when ammonia was used. Information on the particle sizes produced was minimal, but scanning electron microscopy (SEM) pictures showed an average size of about 0.5-1.0 .mu.m for the 20 second residence time product. Purity was found to increase with residence time, the amount of aluminum gradually decreasing in the product with increasing residence time.
Ishizaki et al. (1990), made ultra-fine nitrides, including AlN, using a plasma furnace that vaporized the aluminum and then reacted it with NH.sub.3. The particles were about 50 nm in diameter, and XRD revealed trace amounts of aluminum.
Kimura et al. (1988) presented experimental results discussing the effects of reactor temperature and flow rate of reactants on aluminum nitride particle size. This work indicated that increasing temperature resulted in a narrower size distribution and that particle size became smaller with increasing flow rate. The temperature effect was attributed to increased nucleation rate with temperature, but no explanation was offered for the effect of flow rate. It was probable that this was due to shorter residence times, allowing less time for particle growth. This work was extended (Kimura et al., 1989) to include thermodynamic reasons for this choice of reaction system and to provide more information on the product powder. XRD analysis of the powders revealed only AlN. Uniform spherical particles were found to be formed at temperatures of 1373 K. or greater, as opposed to a mixture of rod shaped and spherical particles at lower temperatures. Unfortunately, the powders were deposited on the reactor walls instead of nucleating homogeneously.
In his thesis entitled "Synthesis of Aluminum Nitride by Nitridation of Aluminum Metal In an Aerosol Flow Reactor", Hashman (1992) studied the production of aluminum nitride powder using the Al/N.sub.2 /Ar system. This system was utilized because of a larger high purity AlN region than other systems such as the AlCl.sub.3 /NH.sub.3 /N.sub.2 system. Hashman preferred the Al/N.sub.2 /Ar system over rite Al/NH.sub.3 /Ar system because it produced greater conversion and product purity in Hashman's configuration. Hashman also taught that the aluminum vapor was introduced upstream of the nitriding gas to prevent surface reaction and nitridation of the solid aluminum used as a source of aluminum vapor.
U.S. Pat. No. 5,126,121 to Weimer et al. discloses a process for the manufacture of aluminum nitride powder having a surface area ranging from 2 to 8 m.sup.2 /g and an oxygen content of less than 1.2 weight percent, by rapidly heating powdered aluminum in a nitrogen atmosphere.
U.S. Pat. No. 5,219,804 to Weimer et al. discloses a process for the manufacture of aluminum nitride powder having a surface area greater than 10 m.sup.2 /g, by nitriding powdered aluminum metal, alumina and carbon or mixtures of powdered aluminum metal and a compatible fine ceramic powder.
Itatani et al. (1993) describe the synthesis of AlN powder by chemical vapor deposition of vaporized aluminum with ammonia and/or nitrogen at low pressures (below 1 k Pa). The specific surface area for a pure nitrogen system is 8.1 m.sup.2 /g, increasing with increasing ammonia content to 77.5 m.sup.2 /g (at 40% ammonia).
Pratsinis et al. (1989) generally describe material synthesis in aerosol reactors, defining aersol reactors as "systems in which particulates are made by gas phase chemical reactions."